1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) element, and a display apparatus, an image processing apparatus, a lighting apparatus, and an image forming apparatus each using the element.
2. Description of the Related Art
In recent years, an organic EL element that spontaneously emits light at a driving voltage as low as about several volts has been attracting attention. The organic EL element has excellent features such as surface emitting characteristics, a light weight, and visibility. Accordingly, the element has been used as a thin display, lighting equipment, or a head-mounted display in order that those excellent features may be exploited.
One problem to be solved in the field of the organic EL element is an improvement in light extraction efficiency. Although a general organic EL element adopts a structure in which an emission layer and an organic functional layer functionally separated from the emission layer are present between a pair of electrodes, the light extraction efficiency of the organic EL element is said to be about 20% to 30%. One cause for the foregoing is as described below. Total reflection may occur at an interface between the emission layer contributing to light emission and the air because the refractive index of the emission layer, which is about 1.7 to 1.8, is higher than the refractive index of the air. Accordingly, light to be actually extracted into the air is limited.
In Japanese Patent Application Laid-Open No. 2012-134128, it is described that even light that has heretofore undergone total reflection is collectively extracted by providing a light extraction member (light extraction unit) such as a lens, which changes the direction of light, in order to improve the light extraction efficiency of an organic EL element. It is also described that an optical path between an emission layer and a metal electrode is specified to the minimum order of interference as an improvement in display performance.
By the way, the behavior of light in an organic EL element can be calculated by an optical simulation described in, for example, S. Nowy et. al., Journal of Applied Physics 104, 123109 (2008). It has also been known that the reflectance, transmittance, phase shift, and the like of an optical multilayer thin film can be calculated (see I. Gontijo et. al., Physical Review B, Vol. 60, (1999), 11564).
Total reflection due to a high refractive index of an emission layer has heretofore been considered to be a main cause limiting the light extraction efficiency of an organic EL element. However, a recent research has pointed out that when the emission layer and a metal film (light reflective electrode) are close to each other, energy from an exciton is coupled to a plasmon on a metal surface, whereby the energy from the exciton is lost. In particular, when an optical path between the emission layer and a metal electrode is set to coincide with the minimum order of interference, the energy loss resulting from the coupling to the plasmon enlarges. In addition, it has been found that the energy loss due to the plasmon is not alleviated even by providing a light extraction unit.